In many applications of direct current to alternating current converters (DTAC) high reliability is a critical feature. Applications of DTAC includes Uninterruptible Power Supplies (UPS), Inverters for Photovoltaics (IPV), windpower systems and micro turbine power generating systems which are becoming more important as business operations and others seek independence from local electrical power grids.
Typically, the reliability of the DTAC is dependent on a relatively small number of components that have a failure physics that limits the lifetime to cost tradeoff to an unsatisfactory level. For example, in IPV systems the time to failure is typically dominated by the failure of the electrolytic capacitors that sit between the switches that provide the time varying AC and the DC current output from the photovoltaic cells. Hence, the electrolytic capacitors experience a substantial AC current typically referred to as “ripple current” which provides the main wear out mechanism for electrolytic capacitors. The equivalent circuit model for the electrolytic capacitor includes a series combination of a resistor (Equivalent Series Resistance or ESR) and the capacitance. The I2R losses associated with the ESR result in self-heating of the electrolytic capacitor which degrades the electrolyte and raises the ESR. This eventually results in a runaway condition causing failure of the electrolytic capacitor which may result in destruction of other components of the DTAC.
Efforts to address the problem of electrolytic capacitor include improving the reliability of the electrolytic capacitors. These efforts are relatively mature at this point which suggests that future improvements in electrolytic capacitor reliability will be small. However, for reasonable size and cost, the reliability of the electrolytic capacitor effectively provides the limiting factor on the lifetime of electronic assemblies built around it. For example, modem IPV systems are typically offered with a five year warranty while the mean time between failure is on the order of six to eight years. This contrasts with the typical warranty for PV panels which are in the range from about 25-30 years.
Improvements in engineering have been largely incremental and design oriented. For example, the requirement that only three-phase operation is provided for DTAC having high power ratings reduces ripple current by balancing the phase loads. However, this acts as an arbitrary constraint on use conditions to improve reliability, and is only completely effective if the three-phase loads are completely balanced. Cooling of the capacitors extends the working life substantially, leading to various heatsink arrangements and forced airflow while increasing noise and power consumption. Other examples include active ripple current management (JP 11-332289) and DTAC designs for minimum capacitance (U.S. Pat. No. 6,804,127).
A secondary failure mechanism is failure of the switching mechanisms typically present in DTAC. The power switches are typically field effect transistors (FET), insulated-gate bipolar transistors (IGBT), gate turn-off devices (GTO) or emitter turn-off switches (ETO). A typical failure mechanism for power switches is thermo-mechanical stress. For example, thermo-mechanical stresses in conventional IGBT devices can cause failure from wire bond fatigue, substrate cracking, and solder fatigue during the operational lifetime of the device.